A glow in the forest

A soft green glow caught my eye the other night, a tiny patch of light in the grass which I recognised as that produced by a female Glow-worm. Soon after, I found other tiny lights, just a handful but a welcome sight nonetheless. Shinning my head torch toward each glow in turn revealed the plump outline of a female Glow-worm, sometimes alone, sometimes accompanied by one or more males. I encounter these insect beacons less frequently these days, a reflection of their general decline within the wider countryside. Gone, too, are the large gatherings of dozens of females grouped together in the vegetation, collectively giving the appearance of a miniature townscape by night.

The light is used by the wingless females to attract a mate, the males being fully winged and sporting the hard wing cases so typical of beetles.  For these are beetles and not, as the name might suggest, worms. In North America they are often referred to as ‘fire-flies’, another misnomer. Globally, there are many different species of glow-worm, although we only have two (possibly three) species in Britain and only one of these is widespread in its distribution and likely to be encountered.

The Glow-worm’s light is the result of a string of controlled chemical reactions which take place within special light organs located in the female’s abdomen. The two most important compounds involved are luciferin and luciferase, the latter acting as a catalyst by bringing together other molecules and holding them in place until a reaction takes place. The reactive process is extremely efficient, wasting less than two per cent of the energy used; compare this with one of the standard light bulbs, now being replaced by low-energy equivalents, which waste 96 per cent of the energy used. A similar process occurs in certain jellyfish, bacteria and even fungi. The light that is produced is a yellow-green colour, which is handy for the human observer as it falls within that part of the light spectrum to which the human eye is most sensitive.

Each species of glow-worm emits its own distinctly-coloured light, effectively a ‘call sign’, which prevents males from approaching a female of a different species. However, a North American glow-worm called Photuris versicolor has been discovered to impersonate the ‘call signs’ of several related species. Males of these other species, fooled into visiting a female Photuris versicolor, may find themselves on the menu!

The almost magical light produced by these insects has attracted our attention for many hundreds of years. In fact, references to glow-worms appear in literature extending back over two thousand years, with, for example, a mention in an ancient Chinese encyclopaedia produced by a pupil of Confucius.